1. Field of the Invention
This invention pertains to classifying, separating, and sorting solids generally, and more specifically in one manifestation to a device using aqueous suspension, sifting, and stratifying to wash and classify aggregate.
2. Description of the Related Art
Cleaning and classifying aggregate matter is an old technology with many applications in modern society. A primary application is in the production of suitable aggregate for the fabrication of high-quality concrete products. As is known in the concrete industry, many characteristics of concrete can be greatly enhanced through the addition of appropriate aggregate materials. The aggregate additives are incorporated in relative quantities based upon particular size ranges, so that a typical mix will include some combination of both sand and one or more sizes of larger stones. Through appropriate size and even shape selection, the concrete can be designed to have the best characteristics for a given application.
However, in order for the aggregate additives to provide the intended benefits without undesirable disadvantages, the aggregate needs to be both clean and properly classified into size ranges. Humus, clay, wood and paper, and even softer and lower density rock such as shale can very adversely affect the performance of a concrete product. These undesirable materials can deleteriously alter such characteristics as compression strength, spalling, and wear or abrasion resistance, chemical resistance, and other characteristics. Consequently, it is very desirable to remove such materials prior to the aggregate matter being incorporated into a concrete mix.
There are many additional applications for washed and classified aggregate, including but not limited to road, building and other construction, landscaping, mining, sand blasting and casting, and even filtration. The different applications may have more or less stringent requirements for both size ranges and cleanliness of product, which may often vary not only by the application, but also by a given job requirement. Consequently, while aggregate for concrete mixes are discussed for exemplary purposes herein, it will be understood that other applications are contemplated as well.
In order to produce suitable aggregate, many facilities depend entirely upon large agitated screens. These screens will typically be loaded with a quantity of aggregate mix, and then shaken or vibrated relatively violently. Matter which is smaller than the screen opening will pass through the screen, where it will typically be caught upon the next screen, which will normally have an even smaller opening size. With sufficient agitation, all of the smaller particles will pass through the screen, while the larger particles will be blocked by the screen. By cascading several screens, it is possible to classify the aggregate mix into particular rock and gravel sizes. Unfortunately, this approach generates a great deal of dust during the agitation of the screens, and so water sprays are often used to keep the dust down. In some instances, the water spray may also assist with the removal of silt or humus, though quite frequently it will be desirable to keep the mist fine and light enough to only serve as dust control. More water may actually interfere with the screening, and may permit clay, for exemplary purposes and not limited thereto, to stick directly to desirable rock. Consequently, without a full washing, the spray can interfere with the separating process. As a result, the classified aggregate produced by this method tends to be relatively dirty, and may require further washing for the more demanding applications.
The use of a water mist also limits the environment where the apparatus may be applicable. As residents of northern climates will recognize, it is not practical to spray a mist during colder, sub-freezing weather. Consequently, the mist dust control is dependent upon warmer weather, undesirably limiting the screening and classifying to the warmer seasons. Further, the mist will rapidly evaporate from the surfaces of the aggregate. This evaporation may lead to very undesirable losses in the very arid climates, again limiting the application and generally preventing misting in arid climates or during times of drought. Since sensitivity of machinery to weather is almost always disadvantageous, causing interruptions in work projects and disruption of schedules, it is consequently desirable to reduce the sensitivity of the apparatus to climate.
Because the screens rely upon lifting and dropping of the aggregate upon the screen, the process requires substantial machinery to have high throughput of matter. In other words, it takes a great deal of energy to repetitively lift and drop the aggregate, and that in turn means large motors and strong frames and supports. Moreover, the extra energy is usually dissipated in the screens, resulting in substantial erosion of the screens and frequent replacement.
Yet another drawback of the agitated screen is the inability of the process to separate out the hardness or density of the materials being sifted. In other words, it is difficult to separate wood and sticks from rocks, and also low-density rocks such as shale from higher density harder rocks. Consequently, when using a sifting process, a separate and additional machine and process is required to further clean and separate undesirable matter.